Polyurethanes that may be crosslinked or cured by radiation to provide a crosslinked polymer are useful, for example, in magnetic recording media wherein the magnetic coating contains a magnetic material, usually a ferri-magnetic material, embedded in a binder matrix of such polyurethanes and small amounts of other additives such as lubricants, dispersants, conductive agents and the like. However, use of many polyurethanes is limited because of processing problems, particularly dimensional stability during processing. Radiation curable polyurethanes having improved and more flexible processability are desired.
U.S. Pat. No. 4,255,243 discloses photo-crosslinkable unsaturated poly-(carbonate-urethanes) obtained by condensing a diisocyanate with a polycarbonatediol and a lower hydroxyalkyl acrylate. U.S. Pat. No. 4,260,703 discloses radiation curable urethane-acrylates prepared by reacting an alkoxylate of an ester diol such as 2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate with a diisocyanate, capped with a hydroxyalkyl acrylate such as hydroxyethyl acrylate. U.S. Pat. No. 4,264,752 discloses compositions formed by the reaction of a polycarbonate polyol, a polyisocyanate and a hydroxy acrylate monomer as hydroxyethyl acrylate that can be used as radiation curable coatings. Also U.S. Pat. No. 4,254,230 discloses unsaturated polyetherester urethanes which are actinic radiation-curable in the presence of air. These are complex polymers containing at least one unsaturated urethane oligomer, said oligomer comprising the reaction product of at least one unsaturated active hydrogen-containing compound, at least one polyisocyanate, and at least one polyetherester. The amount of said unsaturated compound being present in molar amount equivalent to the isocyanate compound. None of these compounds is completely satisfactory as to processability for use in radiation curable magnetic tape applications.